A considerable portion of the world's hydrocarbon endowment exists in carbonate geological formations, while other oil-bearing and natural gas reservoirs are found in sandstones, coalbeds, salt layers, or shales. As these oil and gas reservoirs mature during production, it becomes increasingly difficult to recover the residual hydrocarbons that they still contain. The recovery of hydrocarbons from mature reservoirs tends to be low for two reasons: the reservoirs have low porosity, and the reservoir rock is oil-wet. The combination of these factors means that when oil and gas flow towards the wellbore for production to the surface, there is diminished flow of the oil and gas. The oil-wet and reduced porosity conditions also reduce productivity when enhanced oil recovery (EOR) techniques are employed, such that the injected EOR fluids will bypass the oil that resides in the rock matrix. This problem is not limited to mature reservoirs. During conventional, primary or secondary production of hydrocarbon fluids, the reduced porosity and oil wet conditions reduce the productivity of the well. For example, formation damage occurs by deposition of asphaltenes, condensates, and waxes. The prevention of this type of deposit would avoid the loss of productivity.
The oil-wet state of the reservoir can be caused by asphaltene deposition. Asphaltenes, naturally-occurring components of crude oil, are a complex mixture of aromatic structures that have carboxyl functional groups, characterized, for example, by being insoluble in heptane. Their low solubility leads to their precipitation and deposition onto rock surfaces, causing an oil-wet condition. The permeability of the pores in the carbonate formation is thus reduced as the asphaltene on the surface rock plugs the pore throats. In addition to asphaltenes, gas condensates and waxes form during production, creating an oil-wet condition on the surfaces and in the pores of the petroleum-bearing geologic formation. Gas condensates are liquid or waxy organic materials that deposit in the near-wellbore region, including the fractured zones and proppant packs, of gas producing wells. Waxes are heavy oil fractions or paraffinic materials that can adhere to surfaces.
Removing asphaltenes, condensates, and waxes from oil-containing formations remains an important challenge for oil and gas recovery operations, especially for mature wells where enhanced oil recovery techniques are to be employed. Current approaches to this problem include: (1) dissolving the asphaltenes by treatment with aromatic solvents such as xylene or toluene, or by treatment with dendrimeric polymers; (2) injecting chemicals into the formation to prevent asphaltene deposition; and (3) altering the wettability of the formation. The first two technologies provide only temporary solutions, because they do not slow down the ongoing process of asphaltene deposition. These technologies are also expensive, and incomplete in their ability to treat the entire formation at one time. By changing the wettability of the formation from oil-wet to water-wet, it may be possible to effect a more durable change. Treatment fluids for changing the wettability have been proposed that include, e.g., lignosulfates (see, for example, U.S. Pat. No. 6,051,535) or nanoparticles (see, for example, U.S. Pat. No. 4,579,572). However, conditions in the reservoir are harsh and inhospitable to chemical manipulations. Moreover, permeability and porosity of the well should be preserved or enhanced with any proposed treatment. Therefore, there remains a need in the art for well treatment methods that can enhance the recovery of oil from existing oil reservoirs, while avoiding deleterious effects on the recovery efficiency of the formation.
Oil recovery can also be enhanced by treating the wellbore itself, by treating the formation in workover operations, or by treating the formation as fracturing fluid (“frac fluid”) is installed. The wall of the wellbore can become clogged with fluids that have contacted it, such as asphaltenes, waxes and other hydrocarbon materials from the well, and wellbore oil that has been used during drilling processes. The formation matrix, especially the near wellbore area of the formation, can also become clogged with such deposits. Paraffin deposits can form because of a decrease in temperature below the wax appearance temperature of the oil. Asphaltene deposits can form because of agglomeration, oxidation or chemical interaction of maltene content with incompatible fluids, or pressure or temperature changes. Such deposits can develop in the wellbore and in the formation throughout the lifecycle of the well. Treatments for these organic deposits have included various aromatic solvents and surfactants, as well as high intensity acoustic sources, injection of deasphalted oil, cleaning with carbon dioxide, use of microorganisms, etc.
Acids have been used for oil well treatments aimed at improving productivity. For example acid fracturing is a commonly used technique for increasing the production of oil from a carbonate geological formation. During the acid fracturing treatment a fluid is introduced at sufficient pressure to form fractures in the formation. Acid is then injected into the fracture to chemically react and etch the face of the fracture. The result is that channels are created and the productivity of the well is increased.
Acids are also used in acid treatment aiming at restoring formation damages. In this case the acid is used in areas near the wellbore to reverse the formation damage and restore well productivity. Another application of acids is to reverse the wettability of the formation from oil to water wet. This is particularly relevant in carbonate formations where the asphaltenes tend to deposit on the carbonate surface forming a strong interaction. The oil wet state results in a low success in enhanced oil recovery technique because the injected fluids tend to bypass the oil. Acid has shown to be useful in these situations where the chemical reaction between acid and carbonate help to detach the oil from the rock. Currently several acids are used for these applications including: mineral acids (hydrochloric acid, hydrofluoric acid, or their mixtures); organic acids (acetic acid, formic acid, chloroacetic acid, sulfamic acid, etc.). However these acids tend to be precipitated or consumed relatively quickly and near the wellbore, losing their ability to react deep into the formation or treat surfaces for extended periods of time. One solution is to protect the acid in such a way that it will not be rapidly precipitated or consumed near the wellbore.
There remains a need in the art, therefore, for technologies to improve the cleanout of near wellbore oily deposits to enhance flow of oil into the wellbore. In addition, for reservoirs where hydraulic fracturing is employed, it is desirable to prevent the buildup of hydrophobic residua on the rock surfaces and on the proppant materials. Preventing the accumulation of such residua can improve the efficiency of the hydraulic fracturing endeavor and can improve well productivity.